1. Field of the Invention
The present invention relates generally to the field of oxygen barrier polymers. More particularly, it concerns oxygen barrier compositions and packaging articles comprising a copolymer that comprises (1) repeating units derived from vinylidene chloride and (2) repeating units comprising an oxygen scavenging moiety.
2. Description of Related Art
Due at least in part to their ability to be fabricated into packaging articles of various sizes and shapes (e.g., their design flexibility), plastic materials, such as organic polymers, are becoming increasingly important in the packaging industry. The use of organic polymers in films, bottles, coatings, and liners has become commonplace in packaging. However, while the use of organic polymers has expanded in many areas, their usefulness has been limited in packaging requiring good barrier properties to atmospheric gases (primarily oxygen). This has been because the barrier properties of available organic polymers have not been able to approach the barrier properties of traditional packaging materials, such as glass and metal. New materials having enhanced barrier performance, while retaining benefits inherent to organic polymers would be useful both to the packaging and plastic manufacturing industries.
Organic polymers can be classified by how permeable they are to gases (e.g., oxygen or carbon dioxide) and moisture vapor. Those organic polymers that significantly restrict the ability of gases to pass through them are referred to as gas barrier polymers or high barrier polymers. The use of high barrier polymers is very important in the packaging of certain foods and beverages, which require protection from oxygen and moisture. Vinylidene chloride based polymers (PVDC) and copolymers were discovered some time ago, and they are among the most widely used high oxygen barrier resins. Perhaps the most familiar examples of the vinylidene chloride based polymers used in packaging are commercial Saran(copyright) products. Other high oxygen barrier polymers also widely used, include ethylene vinyl alcohol (EVOH) copolymers. However, EVOH copolymers lack the moisture resistance properties associated with vinylidene chloride based polymers, and therefore, EVOH copolymers are often combined with additional moisture barrier polymers in packaging structures. Packaging comprising high gas barrier polymers, such as PVDC and EVOH, can be inadequate in protecting certain packaged oxygen sensitive products, such as beer and juice, from environmental oxygen. This has prompted extensive efforts to modify organic polymers or packaging structures made from them to enhance oxygen barrier properties. Such efforts have included chemical modification of organic polymers (e.g., fluoridation or sulfonation of organic polymers), and structural variation in packaging design.
For example, xe2x80x9chybridsxe2x80x9d having improved gas barrier performance have been developed. The hybrids involve thin-layer surface coating or deposition of silica or graphite over an organic polymer substrate. Other developments have involved inorganic-organic alloys (nanocomposites) comprising gas impermeable inorganic fillers dispersed in an organic polymer matrix. This technology has resulted in enhanced gas barrier performance as the result of the xe2x80x9ctortuous path effect,xe2x80x9d however these approaches have had limited commercial success due to the cost associated with the additional fabrication steps involved. Thus, it would be desirable to be able to structurally modify PVDC based polymers in such a way that their oxygen barrier performance approaches that of traditional materials, such as glass and metal, for use in more demanding packaging applications.
PVDC based polymers are among the best oxygen barrier polymers, but as discussed above they can be inadequate for use in certain packaging applications for oxygen sensitive food and beverages. PVDC homopolymer is rarely used as a packaging material by itself due to its narrow melt processing temperature. However various PVDC copolymers that comprise small amounts of a comonomer, such as vinyl chloride, methylacrylate or acrylontrile have had some commercial success. While these copolymers offer the desired melt process capability due to their reduced melting points, their gas barrier performance is compromised due to their decreased crystallinity and due to dilution of the vinylidene chloride. Thus, from both a technological and commercial point of view, it would be highly desirable to enhance the gas barrier properties of polymers that are commercially available (e.g., PVDC), while improving their processability.
Another approach to providing packaging for oxygen sensitive products involves inorganic and/or organic oxygen scavengers being used in packaging structures to eliminate or reduce the oxygen inside a package. Oxygen scavengers that can be used include iron powders and unsaturated olefinic polymers. In oxygen scavenging packaging, oxygen within the package or that diffuses through the packaging wall from the outside environment is removed by the irreversible reaction of the oxygen scavenger with the oxygen. In the case of polymeric oxygen scavengers, the oxygen scavenging functionality can be an unsaturated olefinic moiety, which can be incorporated into or grafted onto a polymer. Because these polymers were developed to aid in the removal of headspace oxygen from a package, they typically have relatively high oxygen transmission rates, so that the oxygen within the headspace can easily reach the reactive site (scavenging site) and so that it is reacted at a sufficiently rapid rate. Therefore, most oxygen scavenging polymers used in packaging, such as polyolefins and acrylate polymers, have relatively high oxygen transmission rates and/or relatively low oxygen barrier properties. Oxygen scavenging polymers obtained by grafting cycloalkenyl molecules onto an ethylene-methylacrylate polymer via a reactive extrusion process (Ta Yen Ching et al., Patent Application WO99/48963) represent a significant development in the field of oxygen scavenging packaging. An important advantage of this type of oxygen scavenging polymer is that the oxygen scavenging functionality is based on a cycloalkenyl moiety, which does not result in volatile by-products being formed from the oxygen scavenging reaction.
In the development of new packaging materials, the properties of processability and product performance have typically been found to be interrelated, so that often a compromise has had to be reached favoring one property over the other. This is exemplified in the way in which commercial PVDC copolymers have evolved through the chemical modification of PVDC homopolymer. Commercially successful PVDC copolymers have been made by incorporating small amounts of a comonomer into PVDC polymers improving their processability, however the new copolymers have reduced gas barrier properties as compared to PVDC homopolymer.
The present invention is directed to a novel approach to chemically modifying the PVDC polymer structure that achieves both enhanced oxygen barrier performance and good processability (due to decreased melting point). The present invention is based on the surprising discovery that an oxygen-scavenging vinyl cycloalkenylacrylate monomer can be successfully incorporated into the polymer structure of PVDC by free radical polymerization. The oxygen scavenging vinyl cycloalkenylacrylate monomer has two carbon-carbon double bonds, one in the vinyl moiety of the monomer and the other in the cycloalkenyl moiety of the monomer. The polymerization of vinylidene chloride with the cycloalkenyl acrylate monomer is believed to proceed primarily through reaction of the vinyl double bond of the oxygen scavenging monomer with the carbon-carbon double bond of the vinylidene chloride. Reaction of vinylidene chloride with the double bond of the cycloalkenyl moiety of the oxygen scavenging monomer is believed to be minimal during the polymerization reaction.
The successful incorporation of the vinyl cycloalkenylacrylate into a PVDC polymer results in a polymer, which in the presence of an oxidation catalyst (e.g., cobalt salts), is capable of intercepting or scavenging oxygen as it passes through the composition. The polymer of the present invention is different from conventional oxygen barrier polymers (e.g., PVDC copolymers and EVOH as described above), which act as passive oxygen barriers that are meant to hermetically restrict the diffusion of oxygen into the packaging structure. Another significant aspect of certain compositions of the present invention is that their melting points are reduced sufficiently that their process temperature range is improved. Thus, certain compositions of the present invention can provide improved oxygen barrier properties, while at the same time introducing a desirable reduction in the melting point for easier processing than is typically encountered with other PVDC copolymers. Other benefits of certain methods and compositions of the present invention include the following:
a) improved cost effectiveness, because only a small amount of monomer having oxygen scavenging functionality would be required in PVDC copolymers of the present invention for intercepting oxygen that diffuses from the headspace or from the environment external to the package, resulting in a high gas barrier;
b) optical clarity of PVDC copolymers of the present invention is ensured, because the oxygen scavenging functionality is chemically bonded to the PVDC polymer (this is an improvement over the approach in which an oxygen scavenging component is blended with structural polymers, such as MXD6 in PET, which can result in poor optical clarity);
c) improved barrier properties in packaging applications involving high humidity/high temperature due to the scavenging capacity present in PVDC copolymers of the present invention (this overcomes some of the problems associated with certain high barrier oxygen barrier copolymers, such as EVOH copolymers, as discussed above); and
d) reduction in volatile by-products of the oxygen scavenging reaction, since the reaction of oxygen with the cycloalkenyl ring structure of the oxygen scavenging comonomer does not result in fragmentation by-products (this overcomes the problems associated with some oxygen scavenging polymers, such as polybutadiene, which generate volatile oxidation by-products during scavenging).
In one embodiment, the present invention is directed to an oxygen barrier composition that comprises an oxygen barrier copolymer. Preferably the composition also comprises an oxidation catalyst. The oxygen barrier copolymer in turn comprises repeating units having the formula (I), and repeating units having formula (II). The composition comprises at least about 50 mole % of repeating units having formula (I) and at least about 2 mole % of repeating units having formula (II). 
In formula (II), R is hydrogen or methyl; X is xe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94, xe2x80x94(Cxe2x95x90O)xe2x80x94NHxe2x80x94, xe2x80x94Oxe2x80x94; t is 0 or 1; s is an integer between 0 and 12, inclusive; and J is a cycloalkenyl group. Preferably the cycloalkenyl is cyclohexenyl, a substituted cyclohexenyl, norbornenyl, or a substituted norbornenyl. Preferably substituted cyclohexenyls and substituted norbornenyl have methyl groups as substituents moieties. In a preferred embodiment, the cycloalkenyl is a chemical group having the formula (VI) 
Q is xe2x80x94(CR2R3)nxe2x80x94, wherein n is an integer from 0 to 3, inclusive. Z is xe2x80x94(CR4R5)exe2x80x94, wherein e is an integer from 0 to 3, inclusive, and n+exe2x89xa63, q0, q1, q2, q3, q4, r, each R2, each R3, each R4, and each R5 are independently selected from hydrogen, linear C1-C20 alkyls, branched C1-C20 alkyls, cyclic C1-C20 alkyls, polycyclic C1-C20 alkyls, aromatic groups, halogens, and sulfur-containing substituents, and when r is hydrogen at least one of q1, q2, q3, and q4 is hydrogen. Preferably the group is cyclohexenyl.
In another preferred embodiment, the cycloalkenyl is a group having the formula (VII) 
q1, q2, and r are independently selected from hydrogen, linear C1-C20 alkyls, branched C1-C20 alkyls, cyclic C1-C20 alkyls, polycyclic C1-C20 alkyls, aromatic groups, halogens, and sulfur-containing substituents, and when r is hydrogen at least one of q1, and q2 is hydrogen.
The oxygen barrier copolymer that is described above and that is a component of the oxygen barrier composition comprises at least about 50 mole % of repeating units having formula (I) and at least about 2 mole % of repeating units having formula (II). More preferably the oxygen barrier copolymer comprises at least about 5 mole %, and even more preferably, between about 5 mole % and about 30 mole % of repeating units having formula (III). In certain embodiments, the oxygen barrier composition further comprises a photoinitiator and/or an antioxidant.
In certain embodiments oxygen scavenging repeating units of the oxygen barrier copolymer can have a formula other than formula (II). Such oxygen scavenging repeating units comprise an oxygen scavenging, benzylic structure, such as benzyl acrylate or benzyl methacrylate.
In certain embodiments, the oxygen barrier composition can comprise a polymer blend comprising an oxidation catalyst, the oxygen barrier copolymer described above, and polyvinylidene chloride polymer (e.g., PVDC homopolymer or PVDC copolymer comprising comonomers such as methyl acrylate, methyl methacrylate, acrylonitrile, or vinyl chloride). In such cases, the mole % of the repeating units having the oxygen scavenging functionality of structure (II) can be increased in order to compensate for any dilution effects resulting from blending with other polymers. Preferably, at least about 5% by weight of the oxygen barrier copolymer is oxygen scavenging repeating units in the blend composition.
Yet another embodiment is directed to a method of preparing an oxygen barrier composition comprising, blending polyvinylidene chloride polymer (e.g., homopolymer or copolymer) with an oxidation catalyst, and an oxygen barrier copolymer, as described above. In certain embodiments the oxygen barrier composition can further comprise a photoinitiator. The oxygen barrier copolymer comprises at least 50 mole % of repeating units having formula (I) and at least about 2 mole % of repeating units having formula (II). More preferably the oxygen barrier copolymer comprises between about 5 mole % and about 30 mole % of repeating units having formula (II).
Another embodiment of the present invention is directed to a method of preparing an oxygen barrier copolymer as described above, comprising reacting a plurality of monomers to produce an oxygen barrier copolymer. The plurality of monomers reacted comprise vinylidene chloride monomers and vinyl monomers having formula (III). 
R is hydrogen or methyl; X is xe2x80x94(Cxe2x95x90O)xe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94(Cxe2x95x90O)xe2x80x94, xe2x80x94(Cxe2x95x90O)xe2x80x94NHxe2x80x94, xe2x80x94Oxe2x80x94; t is 0 or 1; s is an integer between 0 and 12, inclusive; and J is a cycloalkenyl group. Preferably the cycloalkenyl is cyclohexenyl, a substituted cyclohexenyl, norbornenyl, or a substituted norbornenyl. Preferably substituted cyclohexenyls and substituted norbornenyls have methyl groups as substituents moieties. In a preferred embodiment, the cycloalkenyl is a chemical group having the formula (VI) or formula (VII), as described above.
Preferably the cycloalkenyl oxygen scavenging moiety of formula (III) is cyclohexenyl. The plurality of monomers that is reacted to produce the oxygen barrier copolymer comprises at least about 50 mole % of vinylidene chloride monomers and at least about 2 mole % of vinyl monomers having formula (III). More preferably the plurality of monomers that is reacted comprises between about 5 mole % and about 30 mole % of monomers having formula (III).
In still another embodiment, the present invention is directed to a packaging article that comprises an oxidation catalyst and at least one oxygen barrier layer comprising an oxygen barrier copolymer, as described above. The oxygen barrier layer can comprise the oxidation catalyst, or the oxidation catalyst can be in a layer adjacent to the oxygen barrier layer. The oxygen barrier layer can in certain embodiments further comprise a photoinitiator. The packaging article can further comprise materials such as antioxidants, among others. In certain embodiments, the oxygen barrier layer comprises a polymer blend comprising polyvinylidene chloride polymer (e.g., homopolymer or copolymer) and the oxygen barrier copolymer described above. The packaging article can further comprise additional layers selected from oxygen barrier layers, structural layers, and oxygen scavenging layers, among others.
Oxygen sensitive materials packaged using oxygen barrier copolymers and compositions of the present invention can have a relatively long useful shelf life, and in certain embodiments, the shelf life can be longer than when conventional oxygen barrier packaging (e.g., packaging prepared using polyvinylidene chloride (PVDC) or EVOH) is used.